Transcript Document
The Eruption of Mount St Helens, 18
th
May 1980
The Cascade Range is a volcanic chain stretching from northern California to British Columbia
The smallest of five major volcanic peaks in Washington State was Mount St. Helens, with an elevation of 2,950 metres before the eruption of May 18, 1980. Another Cascade volcano, Mount Adams, is in the distance. The view is from the west.
A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak. Measurements of the bulge indicated it was growing at a rate of up to 1.5 metres per day. By May 17 th , part of the volcano's north side had been pushed upwards and outwards over 135 metres.
On May 18 th , 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure, and triggering a major pumice and ash eruption of the volcano. 400 metres of the peak collapsed or blew outwards. As a result, 62 square kilometres of valley was filled by a debris avalanche. Fifty-seven people were killed or are still missing.
For more than nine hours a vigorous plume of ash erupted, eventually reaching 20-25 kilometres above sea level. The plume moved eastward at an average speed of 60 miles per hour, with ash reaching Idaho by noon. By early May 19 th , the devastating eruption was over. Shown here is a close-up view of the May 18 th ash plume.
Map of the area around Mount St. Helens which were affected by the May 18, 1980 eruption. North is to the top.
After the May 18 th , 1980 eruption, Mount St. Helens' elevation was only 2,550 metres and the volcano had a 1.5 kilometre horseshoe shaped crater.
Before and after…
For weeks volcanic ash covered the landscape around the volcano and for several hundred miles downwind to the east. Noticeable ash fell in eleven states.
The slopes of Smith Creek valley, east of Mount St. Helens, show trees blown down by the May 18 th , 1980 lateral blast. Two U.S. Geological Survey scientists (lower right) give scale. Over four billion feet of timber, enough to build 150,000 homes, was damaged or destroyed.
Spirit Lake, once surrounded by lush forest, is within the area devastated by blast. Remnants of the forest float on the surface of the lake.
Nearly 220 kilometres of river channels surrounding the volcano were affected by the mudflows (lahars) of May 18 th , 1980. A mudline left behind on trees shows depths reached by the mud. A scientist (middle right) gives scale.
More than 200 homes and over 300 kilometres of roads were destroyed by the 1980 mudflows.
During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the sides of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour and reach temperatures of over 400°c. Photographed here, a pyroclastic flow stretches from Mount St. Helens' crater to the valley floor below.
U.S. Geological Survey scientist examines pumice blocks at the toe of a pyroclastic flow.
Five more explosive eruptions of Mount St. Helens occurred in 1980 (after May 18 th ), including this spectacular event of July 22. This eruption sent pumice and ash 10-18 kilometres into the air, and was visible in Seattle, Washington, 160 kilometres to the north.